.TH std::pmr::null_memory_resource 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::pmr::null_memory_resource \- std::pmr::null_memory_resource

.SH Synopsis
   Defined in header <memory_resource>
   std::pmr::memory_resource* null_memory_resource() noexcept;  \fI(since C++17)\fP

   Returns a pointer to a memory_resource that doesn't perform any allocation.

.SH Return value

   Returns a pointer p to a static storage duration object of a type derived from
   std::pmr::memory_resource, with the following properties:

     * its allocate() function always throws std::bad_alloc;
     * its deallocate() function has no effect;
     * for any memory_resource r, p->is_equal(r) returns &r == p.

   The same value is returned every time this function is called.

.SH Example

   The program demos the main usage of null_memory_resource: ensure that a memory pool
   which requires memory allocated on the stack will NOT allocate memory on the heap if
   it needs more memory.


// Run this code

 #include <array>
 #include <cstddef>
 #include <iostream>
 #include <memory_resource>
 #include <string>
 #include <unordered_map>

 int main()
 {
     // allocate memory on the stack
     std::array<std::byte, 20000> buf;

     // without fallback memory allocation on heap
     std::pmr::monotonic_buffer_resource pool{buf.data(), buf.size(),
                                              std::pmr::null_memory_resource()};

     // allocate too much memory
     std::pmr::unordered_map<long, std::pmr::string> coll{&pool};
     try
     {
         for (std::size_t i = 0; i < buf.size(); ++i)
         {
             coll.emplace(i, "just a string with number " + std::to_string(i));

             if (i && i % 50 == 0)
                 std::clog << "size: " << i << "...\\n";
         }
     }
     catch (const std::bad_alloc& e)
     {
         std::cerr << e.what() << '\\n';
     }

     std::cout << "size: " << coll.size() << '\\n';
 }

.SH Possible output:

 size: 50...
 size: 100...
 size: 150...
 std::bad_alloc
 size: 183
